获取tableView或者collectionView或者web

监听contentSize变化

[self.tableView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:nil];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    CGFloat height = self.tableView.contentSize.height;
//    NSLog(@"亲高度%f",height);
    [self.tableView mas_updateConstraints:^(MASConstraintMaker *make) {
        make.height.mas_equalTo(height);
    }];
}